Aphids and ants

Aphids are among the more familiar insects. They are small, wingless, soft, nearly oval bodied insects, green, yellow or dark in colour. They are seen adhering in groups to leaves, tender stems or fruits of plants. They have long needle like feeding organs or mouth parts, like those of a mosquito. These feeding needles are inserted into the plant tissues to suck plant juices, which constitute their food. They are quite sluggish, and change their position on the plant body of their own only rarely.

Though aphids are mostly wingless and idle feeders on plants, occasionally winged individuals appear among them. The winged aphids help dispersal of the species.

The long feeding needles, with a food channel between them, are penetrated deep into the plant tissues, and the tips of the needles reach a phloem cell. Phloem is the plant tissue made up of columns of dilated cells, through which products of photosynthesis, mainly a sugary solution, flow in the plant body, generally in the downward direction, that is away from the leaf bearing part of the plant. That aphids suck juices from the phloem is a statement not wholly correct. The turgor pressure in phloem cells actually forces fluid nourishment into the insect body through its feeding needles. Thus the idle insect gets its food without much efforts on its own part. There is a sucking pump in the head of an aphid, but, if using a sharp instrument the feeding organs are cut across at their base so that the feeding needles remain in situ penetrated into the plant body, while the rest of the insect including the sucking pump in its head is removed, plant sap is seen coming out from the cut end of the needle like mouth parts, and this continues for days (Zimmermann, 1963). Obviously the sluggish aphid gets its food effortlessly. In fact the constant flow of liquid nourishment into the aphid body brings in more food than is needed. Hence from time to time a drop of a sugary liquid appears at the anal end of the aphid. This sweet drop is called honey dew, and is readily grabbed by ants.

This ready availability of food keeps ants moving among aphid colonies, and this association of aphids and ants has evolved into a remarkable case of mutual benefit or symbiosis. Ants get advantage of getting nourishing food. Often ants are seen stroking the abdomens of aphids with their feelers. This makes aphids give out a larger droplet of the sugary liquid. This behaviour of ants does not seem to disturb or irritate the aphids in any way. The hind legs of the aphids resemble the ant feelers and the aphids frequently swing their hind legs, as if inviting ants for feeding.

Ants get advantage of easy availability of food through association with aphids. At the same time aphids get quite some advantage out of this association. Aphids, being wingless and sluggish, are an easy prey to insect parasites and predators. Ants actively defend aphid colonies from such invaders. Their defensive behaviour includes their quick and energetic movements around aphids, biting movements of their jaws and spurting a poisonous liquid through their anal end. If an attacker is not easily driven away, the ants may form a circle around a group of aphids. Some ants deposit debris or make a silken tent over an aphids aggregate. They may make their nest enclosing aphid colonies. The last defensive step is generally shown by ants associated with root feeding subterranean aphids. Pavilions are built by Oecophylla ants for the protection of their coccids, which also produce a sugary liquid, and Pheidole build them for their aphids and for the paths leading to the aphid groups. The Hippeococcus plant lice (Pseudococ-cidae) in Java, which are also honey dew producing hemipterans, are associated with Dolichoderus ants, and, if they are disturbed or menaced by predators, some of them climb on the back of the ants, whilst other lice are grasped between the ant mandibles (Reyne, 1954) and are thus protected.

Aphids from their association with ants reap another advantage. Perhaps company of ants has made aphids very sluggish. Even if the feeding source at one place has nearly dried up, they would not withdraw their feeding needles and move away. It is their tending ants which move them. The ants very carefully pull out the aphid feeding stylets, and then bodily carry away the aphids to a new site or to a new plant. Aphids do not resist this action; in fact they seem to enjoy this free ride (Poole and Poole, 1963).

In temperate parts ants do not leave aphids to die of cold in winter. When it is very cold and freezing outside, they lift their food providing associates, take them to their subterranean nests, and put them in warmth there against roots and rootlets. The aphids drill their feeding stylets into these parts of a plant, and are cosily lodged for the winter. They are again ready to produce sugary drops for the ants. When it is spring the aphids are removed to more exposed parts of the plant by their tenders (Poole and Poole, 1963).

Dying aphids are also a source of food for ants, as ants, associated with aphids, are carrion feeders too.

Thus ants protect and tend colonies of aphids as man looks after his cows, and aphids offer sugary liquid food to ants, as cows provide milk for human consumption. This symbiotic association has made aphids more helpless and dependent on ants, changes almost amounting to 'domestication'. Aphids have been called ant cows, and aphid tending ants cow herders.

Certain other plant feeding insects are also known to offer sugary food or gland secretions to ants, and get the advantage of their defence in return, for example some lycaenid and riodinid caterpillars, sap feeding Ho-moptera (aphids, coccids, pseudococcids, membracids, etc.) and some weevils. But the extent of 'domestication' in their case is sometimes less than in aphids. Some of these honey dew producers, other than aphids, have been mentioned above.

— Fig. 39.3. A: An aphid feeding position; B: feeding stylets of an aphid, cut across their base, when the insect is feeding. Plant sap droplets keep on appearing for days (based on photographs in Zimmerman, 1963).